Pressure-driven membrane filtration processes have matured and are now widely accepted in many industries such as in energy, biotechnology, food and beverage, chemical, wastewater, gas fractionation, and desalination due to their low energy requirements and one-phase operation. Although membranes made from metals or ceramics are available, polymeric materials predominate (Ulbricht, Advanced Functional Polymer Membranes, Polymer, 2006, 47 (7), 2217-2262). For over 40 years, both interfacial polymerization (Cadotte et al., Thin-Film Composite RO Membranes: Origin, Development and Recent Advances, In Synthetic Membranes; Turbak, Ed., ACS Symposium Series, American Chemical Society. Washington, D.C., 1981; Vol. 1, pp 305-326) and phase inversion (Loeb, The Loeb-Sourirajan Membrane: How It Came About, in Synthetic Membranes, Turbak, Ed.; ACS Symposium Series, American Chemical Society: Washington, D.C., 1981; Vol. 1, pp 1-9) have been the predominant methods for preparing composite polymeric and asymmetric membrane structures, respectively. Although these synthesis methods have been very successful, they are both relatively complex and sensitive to small changes in the casting conditions (Elimelech et al., The future of seawater desalination: Energy, technology, and the environment, Science, 2011, 333 (6043), 712). Many research groups have sought novel synthesis methods for producing synthetic membranes without much success. These membranes suffer from limitations including low porosity (track etched) (Price et al., Observation of Charged Particle Tracks in Solids, J. Appl. Phys., 1962, 33, 3400; Price et al., Chemical Etching of Charged Particle Tracks, J. Appl. Phys. 1962, 33, 3407; Quinn et al., Model Pores of Molecular Dimension: The Preparation and Characterization of Track-Etched Membranes, Biophys. J., 1972, 12 (8), 990-1007), high cost (ceramic or stainless steel) (Anderson et al., Titania and Alumina Ceramic Membranes, J. Membr. Sci., 1988, 39 (3), 243-258), wide pore size distribution (stretched PTFE) (Gore et al., Process for producing porous products, U.S. Pat. No. 3,953,566; Gore et al., Waterproof laminate, U.S. Pat. No. 4,194,041; Gore, Porous products and process therefor. U.S. Pat. No. 4,187,390), and low strength (biological) (Renkin, Filtration, Diffusion, and Molecular Sieving Through Porous Cellulose Membranes, J. Gen. Physiol., 1954, 38 (2), 225-243), or are difficult to scale-up (zeolite, carbon-nanotubes or graphene oxide) (Yoo et al., High-Performance Randomly Oriented Zeolite Membranes Using Brittle Seeds and Rapid Thermal Processing, Angew. Chem., Int. Ed., 2010, 49 (46), 8699-8703; Hinds et al., Aligned multiwalled carbon nanotube membranes, Science, 2004, 303 (5654), 62-65, Nair et al., Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science, 2012, 335 (6067), 442-444).
We developed a new class of synthetic brush hydrophobic polymer membranes, and tested them with a challenging separation: removal of isobutanol from water by pervaporation (PV). To prepare the best performing hydrophobic brush membranes for this separation, we used our unique high throughput platform with 96 filter well plates (Zhou et al., High-throughput membrane surface modification to control NOM fouling, Environ. Sci. Technol., 2009, 43 (10), 3865-71). The method of preparation involves grafting commercially available vinyl monomers alone or in mixtures to light-sensitive poly(ether sulfone) (PES) nanofiltration membranes, screening for the best performers, and selecting the winners (Zhou et al., High Throughput Synthesis and Screening of New Protein Resistant Surfaces for Membrane Filtration, AIChE J., 2010, 56 (7), 1932-1945; Gu et al., High Throughput Atmospheric Pressure Plasma-Induced Graft Polymerization for Identifying Protein-Resistant Surfaces, Biomaterials, 2012, 33 (5), 1261-1270; Zhou et al., High Throughput Discovery of New Fouling-Resistant Surfaces, J. Mater. Chem., 2011, 21 (3), 693-704). The high throughput platform has also been used with combinatorial chemistry to generate a library of new monomers (Gu et al., High Throughput Atmospheric Pressure Plasma-Induced Graft Polymerization for Identifying Protein-Resistant Surfaces, Biomaterials, 2012, 33 (5), 1261-1270; Imbrogno et al., A New Combinatorial Method to Synthesize, Screen, and Discover Anti-Fouling Surface Chemistry, ACS Appl. Mater. Interfaces, 2015, 7, 2385-2392). The newly synthesized hydrophobic-terminated monomers were then grafted and polymerized on the surface of multiple light-sensitive PES membranes located in filter wells in 96 well plates. This high throughput process allows one to rapidly screen many different surface chemistries with reproducibility and high confidence. The selected winners were determined by measuring performance parameters: fouling index, selectivity, and permeation flux (inverse resistance to flow). This method allows one to fine-tune the surface chemistry based on the desired application.
Research on hydrophilic polymer brushes grafted inside of the pores of selective porous supports for pervaporation began in the 1990s (Ulbricht et al., Novel high performance photograft composite membranes for separation of organic liquids by pervaporation, J. Membr. Sci., 1997, 136 (1), 25-33; Yamaguchi et al., Plasma-graft filling polymerization: preparation of a new type of pervaporation membrane for organic liquid mixtures, Macromolecules, 1991, 24 (20), 5522-5527). More recently, non-hydrophobic polymer brush membranes on inorganic supports, first reported by Cohen's group 15 years ago, did not gain traction because they were difficult to prepare and scale-up, and the inorganic substrates were costly (Jou et al., A novel ceramic-supported polymer membrane for pervaporation of dilute volatile organic compounds, J. Membr. Sci., 1999, 162 (1), 269-284; Lin et al., Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance, J. Mater. Chem., 2010, 20 (22), 4642-4652; Yoshida et al., Ceramic-supported polymer membranes for pervaporation of binary organic/organic mixtures, J. Membr. Sci., 2003, 213 (1), 145-157). Specifically, relatively polar polyvinyl acetate polymer was grafted on inorganic silica membranes using a complicated three-step process and used for pervaporation of chlorinated hydrocarbons, such as chloroform and trichloroethylene from water (Jou et al., A novel ceramic-supported polymer membrane for pervaporation of dilute volatile organic compounds, J. Membr. Sci., 1999, 162 (1), 269-284). The differences between the polymer brush membranes prepared on inorganic supports and our new hydrophobic polymer brush membranes are significant, since our method requires less time (1 day vs 4-5 days), does not require harsh solvents or initiating catalysts, lacks complicated chemical modification steps, and uses hydrophobic brushes instead of hydrophilic brushes (Crivello et al., Low fouling ultrafiltration and microfiltration aryl polysulfone. U.S. Pat. No. 5,468,390). Our process is also scalable and less expensive (Zhou et al., High Throughput Synthesis and Screening of New Protein Resistant Surfaces for Membrane Filtration, AIChE J., 2010, 56 (7), 1932-1945; Zhou et al., High Throughput Discovery of New Fouling-Resistant Surfaces, J. Mater. Chem., 2011, 21 (3), 693-704; Taniguchi et al., Photo-processing and cleaning of PES and PSF Membranes, WO 03/078506; Belfort et al., UV-Assisted Grafting of PES and PSF Membranes, CA 2,422,738). Much of the focus since then has been on using grafted hydrophilic brushes to repel proteins and other molecules as an antifouling mitigation strategy (Lin et al., Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance, J. Mater, Chem., 2010, 20 (22), 4642-4652; Malaisamy et al., Development of reactive thin film polymer brush membranes to prevent biofouling, J. Membr. Sci., 2010, 350, 10; Varin et al., Biofouling and cleaning effectiveness of surface nanostructured reverse osmosis membranes, J. Membr. Sci., 2013, 446, 10; Varin et al., Wettability of terminally anchored polymer brush layers on a polyamide surface, J. Colloid Interface Sci., 2014, 436, 286-95; Cohen et al., Membrane Surface Nanostructuring with Terminally Anchored Polymer Chains, In Functional Nanostructured Materials and Membranes for Water Treatment, Duke et al. Ed.; Wiley-VCH Verlag: New York, 2013, p 40; Rahaman et al., Control of biofouling on reverse osmosis polyamide membranes modified with biocidal nanoparticles and antifouling polymer brushes, J. Mater. Chem., B 2014, 2, 8; Cohen et al., Fouling and scaling resistant nanostructured reverse osmosis membranes, U.S. Pat. No. 8,445,076). These hydrophilic fouling-resistant brushes are located above a selective membrane film, are usually nonselective, and only provide a barrier to foulants (e.g., proteins and natural organic matter), while our hydrophobic brushes are the selective layer attached to a nonselective support layer. This and excellent performance of our synthetic membranes are the novel aspects of our invention. In one case, a hydrophilic tethered brush had selective properties for salt rejection, but this is quite different from the selective hydrophobic brush presented here because it is not suitable for recovery of organics with pervaporation (Cohen et al., Fouling and scaling resistant nanostructured reverse osmosis membranes, U.S. Pat. No. 8,445,076).
Pervaporation is a combination of a membrane (rate governed) and thermal (equilibrium) process and is most widely used alone or in combination with distillation. The advantage of using this process is that it easily breaks azeotropes and fractionates closely boiling liquids, in contrast to thermal processes (Baker, Pervaporation. In Membrane Technology and Applications, 3rd ed., Baker, Ed.; John Wiley & Sons: New York, 2012, pp 379-416). An important example is the dehydration of ethanol and isopropanol water mixtures in the pharmaceutical and fine chemical industries. The first step is to distill water from ethanol until the azeotrope is formed (at ˜10% water) and then, in the second step, use pervaporation to yield a final water content of <1%. This purity of ethanol allows it to be used as a fuel. An aqueous stream containing alcohol is passed across a pervaporation membrane allowing the alcohol to dissolve into the membrane and then diffuse down a chemical potential gradient to the second face of the membrane where the alcohol evaporates into a carrier gas or is allowed to re-condense at a cooled surface, while the retained polar component (water in this case) concentrates on the feed side. The difference in chemical potential between the two phases is the driving force for permeation. Passing water through the membrane in preference to alcohol necessitates a hydrophilic membrane while the reverse requires a hydrophobic membrane. Thus, material choice is critical for selectivity of pervaporation membranes, since the mechanism of transport is based on the solution-diffusion model (Lee et al., Sorption, Diffusion, and Pervaporation of Organics in Polymer Membranes, J. Membr. Sci., 1989, 44 (2), 161-181). Rubbery poly(dimethylsiloxane) (PDMS, also called silicone rubber or Sil5 and Sil20 here) is used commercially to selectively pass ethanol in preference to water, and relies on sorption selectivity rather than diffusion selectivity (Blume et al., The Separation of Dissolved Organics from Water by Pervaporation, J. Membr. Sci., 1990, 49 (3), 253-286), while the opposite holds for poly(vinyl alcohol), which is hydrophilic and is both sorption and diffusion selective for water passage (Chapman et al., Membranes for the Dehydration of Solvents by Pervaporation, J. Membr. Sci., 2008, 318 (1), 5-37). These materials plus cellulose acetate have been used in asymmetric or composite structures for the past 30 years. Clearly, new materials with superior performance are needed. We disclose a new class of superior performing hydrophobic brush membranes (i.e., our synthetic membranes) for the selective recovery of isobutanol for use as a biofuel that are simple to prepare, easy to scale-up, and environmentally friendly. Our synthetic membranes are also useful for selective recovery of other volatile organic compounds.
Moreover, the present invention also relates to reverse osmosis processes and purification of water methods. More than 1 billion people worldwide lack access to potable water, and almost 2 million children die each year for want of clean water and adequate sanitation. California is already struggling with drought and water scarcity is predicted to increase in the western states of the United States during the next 30 years. The increase in population, energy needs, and the industrial development of countries like China, India, and Brazil will lead to water supply challenges. Clearly, during the next 30 years, stresses to the available water supply will increase.
Three methods that can increase our water supply include desalination, purification of low-quality water (brackish groundwater and storm water), and purification and reuse of wastewater. Reverse osmosis has moved from cellulose acetate based membranes to hydrophilic polyamide based membranes. Current desalination membrane systems are more efficient than the thermal desalination systems of the past and have improved significantly since their inception approximately 60 years ago. Desalination of brackish and seawater is one of several nontraditional sources of water being considered to supplement current potable water needs and complement water-reuse strategies. Reclaiming, recycling, and reusing water creates new sources of high-quality water supply and addresses these challenges.
As has been reported, the economics of desalination has changed dramatically over the past three decades, with improvements in energy recovery and membrane technology. Desalting brackish groundwater is a growing practice in the United States, and with 96% of the world's water in the oceans, seawater desalination is clearly a major opportunity for future water needs. According to the International Desalination Association, in 2013 there were more than 17,000 desalting units globally, with installed capacity of 21.1 billion gallons per day (80 million m3 day−1). Japan is building a megaton plant (1 million m3 day−1) that is scheduled to open in 2020. Here, we disclose methods of use of our synthetic membranes for purification of water from ionic solutions of water and inorganic ions.
We also disclose methods of use of our synthetic membranes for isolating non-polar gases by gas fractionation. Gas separations operate according to the solution diffusion model or the Knudsen diffusion model. In the solution diffusion model (dense), the selectivity of the gas is dependent upon the diffusion selectivity and the sorption selectivity of the membranes. Smaller gas molecules will diffuse through the membrane faster than larger ones. Also, if the sorption of the gas is high, then the gas will permeate the membrane faster. Sorption is dependent upon the condensability of the molecules transporting through the membrane (called permeant) Larger molecules are usually more condensable and therefore will have a higher sorption into the membrane. In the Knudsen diffusion model (porous), convective flow, Knudsen diffusion, and molecular sieving will govern the selectivity. If the pores are too large (0.1-10 μm), then there will be no selectivity, as in convective flow. If the pores are less than 0.1 μm, they are on the same order as the mean free path of the gas. Then they will follow Knudsen diffusion and the transport rate will be inversely proportional to the square root of the molecular weight. Even smaller pores (5-20 Å) operate under the surface diffusion model and are more complex. More information about gas separation methods can found in Baker R W. 2004. Gas Separation, In Membrane Technology and Applications, pp. 301-53: John Wiley & Sons, Ltd, 2nd edition.